This invention relates generally to vortex-shedding flowmeter transmission systems and more particularly to a system of this type which is useful for both liquid and gas flow rate measurement and has a jitter-free output.
It is well known that under certain circumstances the presence of an obstacle or shedder in a flow conduit will give rise to periodic vortices. For small Reynolds numbers, the downstream wake is laminar in nature, but at increasing Reynolds numbers, regular vortex patterns are formed. These patterns are known as Karman vortex streets. The frequency at which vortices are shed in a Karman vortex street is a function of flow rate.
It is this phenomenon which is exploited to create a flowmeter to measure the volumetric flow of fluids being treated or supplied in order to carry out various control functions. Flowmeters of this type are disclosed in Bird U.S. Pat. No. 3,116,639, and in White U.S. Pat. No. 3,650,152. Existing flowmeters of the vortex shedding type, such as those disclosed in the Burgess U.S. Pat. No. 3,888,120 and the Herzl U.S. Pat. No. 4,162,238, are capable of effecting volumetric or mass flow measurement.
Though some vortex-shedding flowmeters have high accuracies, better than 0.5% of flowrate, or even in some instances 0.25% of flow rate, these accuracies are of little practical value in many situations by reason of the low resolution of the output as well as jitter effects. By "jitter" is meant small, rapid variations in the output as a result of fluid flow disturbances or other factors.
For example, a vortex-shedding flowmeter for liquid flow measurement having a six-inch diameter might have an output whose frequency lies in a range extending between 2 Hz and 30 Hz, depending on flowrate. With a 2 Hz output and with normal counting techniques, 500 seconds are required to obtain 1000 counts or 0.1% resolution. When calibrating this instrument against a volume or weight standard, a standard of adequate size (about 1000 gallons), would be required to contain the fluid passed by the flowmeter during this period.
If period or line measurement is employed, then a time variation of between 2% to 20% per cycle is encountered. This depends on shedder design, piping and installation parameters, and other factors. It therefore still requires a relatively prolonged period to obtain a 0.1% of rate measurement.
It is known that the effects of noise and jitter in a vortex-shedding meter can be minimized and signal quality improved by filtering out frequency components that are not part of the shedding frequency. Thus in the Herzl U.S. Pat. No. 3,709,034, there is disclosed a system including a signal conditioner associated with the output of the vortex-shedding meter. This conditioner is adapted to extract the dominant frequency representing flow rate from the composite output signal frequency and to exclude high and low frequency noise components, whereby by measuring only the dominant frequency, one obtains an accurate reading of fluid flow quantity. But in this known arrangement, the output frequency is within a low frequency range and the resolution of the output is low.
As pointed out in Herzl U.S. Pat. No. 4,123,940, multiplication of the output frequency of a vortex meter is especially important for large meters where the dominant or natural frequencies are very low and where it is therefore difficult to obtain adequate resolution and reasonable time constants without frequency multiplication. But while the arrangement disclosed in this prior patent serves to improve the resolution of the output, it does not reduce jitter. Indeed, severe jitter may actually give rise to multiplication errors.
Another problem encountered with vortex-shedding meters and also with Swirlmeters which yield a low-frequency meter signal whose frequency is a function of flow rate is the generation of spurious signal components even at zero flow rate. These signal components, which are from hydraulic, electrical or mechanical noise, result in inaccurate readings of flow rate.